dart flutter中函数参数(_)

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https://stackoverflow.com/questions/25517016/what-does-it-mean-to-pass-i-e-underscore-as-the-sole-parameter-to-a-dart

2020年09月23日 2,618 次阅读

问题1:I'm learning Dart and see the following idiom a lot:

someFuture.then((_) => someFunc());

I have also seen code like:

someOtherFuture.then(() => someOtherFunc());

Is there a functional difference between these two examples? A.k.a., What does passing _ as a parameter to a Dart function do?

This is particularly confusing given Dart's use of _ as a prefix for declaring private functions.

It's a variable named `_` typically because you plan to not use it and throw it away. For example you can use the name `x` or `foo` instead. The difference between `(_)` and `()` is simple in that one function takes an argument and the other doesn't.

问题2:So, the function being called expects a parameter, and the code is passing junk (an unassigned variable)? Why not use null, for clarity? Just to save typing? And, won't the function be a little surprised when it tries to use the unassigned, untyped variable? Or, am I missing something?

It's not passing junk, it's just not being used. 2) `null` is a keyword and can't be used as a variable name. 3) Yes, It saves typing and also gives a low profile look. 4) No, `_` is an assigned variable. No surprises here.

dart flutter中函数参数(_)

It's a variable named `_` typically because you plan to not use it and throw it away. For example you can use the name `x` or `foo` instead. The difference between `(_)` and `()` is simple in that one function takes an argument and the other doesn't.

It's not passing junk, it's just not being used. 2) `null` is a keyword and can't be used as a variable name. 3) Yes, It saves typing and also gives a low profile look. 4) No, `_` is an assigned variable. No surprises here.

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It's a variable named _ typically because you plan to not use it and throw it away. For example you can use the name x or foo instead. The difference between (_) and () is simple in that one function takes an argument and the other doesn't.

It's not passing junk, it's just not being used. 2) null is a keyword and can't be used as a variable name. 3) Yes, It saves typing and also gives a low profile look. 4) No, _ is an assigned variable. No surprises here.

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`auth()->id()` 到底返回谁？Sanctum 多态令牌与双会员系统的踩坑记录

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flutter的 Clean Architecture架构设计

It's a variable named `_` typically because you plan to not use it and throw it away. For example you can use the name `x` or `foo` instead. The difference between `(_)` and `()` is simple in that one function takes an argument and the other doesn't.

It's not passing junk, it's just not being used. 2) `null` is a keyword and can't be used as a variable name. 3) Yes, It saves typing and also gives a low profile look. 4) No, `_` is an assigned variable. No surprises here.